over debited my account

I received calls for the past two months regarding checkers vouchers, the agreement was that they will debit R89 on a monthly basis and I would receive voucher, they debited my account with R99.99 on the 25th of Feb and R502. 00 on the 1st of March. I want all the money reversed back to my account , I do not want their stupid vouchers anymore. its a *********** scheme where they lie to client over the phone. I never gave in authority for R600 to be debited. I want my money back